Radio communication receivers must receive desired signals at predetermined frequencies while filtering out undesired signals at other frequencies. The undesired signals can have a much larger signal strength than the desired signals. These undesired signals are called “blockers” and are problematic because the filter that removes them must operate in its linear region to avoid distortion. Therefore, the maximum amplitude of all signals applied to the filter input must be limited so that the blocker signals do not cause saturation, clipping or subtler forms of distortion such as intermodulation (a typical measure of which is out-of-band third order input intercept point or IIP3) in the filter. Consequently, the strength of the entire range of signals to be processed, including the blocker signal and the desired signal, must be appropriately limited. This means that the desired signal, which can be orders of magnitude smaller than the blocker signals, will be limited to an extremely small amplitude and can fall below the noise floor of the filter.
To keep the signal-to-noise ratio within a reasonable range, the noise of the filter, which is advantageously implemented as an integrator circuit, must be kept extremely low which can result in a large chip area and have large power dissipation. Accordingly, it is difficult to implement these filters on an integrated circuit chip, necessitating the use of other technologies which can have penalties in terms of cost and size.
One technique known in the prior art to construct a radio communication receiver is the active RC technique as described in Mihai Banu & Yarmis Tsividis, An Elliptic Continuous-Time CMOS Filter with On-Chip Automatic Tuning, SC-20 IEEE Journal of Solid-State Circuits, 1114, 1114-1121 (December 1985). This technique employs fully-balanced integrator stages, each stage consisting of resistors, capacitors and operational amplifiers. Fully-balanced operation, which means that each integrator has two output terminals where the signals at each output terminal are identical in magnitude to one another, but have opposite polarity, improves the filter's common-mode interference rejection performance. Because the frequency response of an active filter designed in this fashion depend on the resistance and capacitance values of the chip components, and because those values may vary due to fabrication tolerances and temperature variations, a technique was described for making the resistors in the active filter tunable to compensate for unwanted variations in the frequency response of the filter. Specifically, the prior art describes all of the resistance element in the active filter being embodied as MOSFETs operating in their triode or nonsaturation region, designed and biased to function as variable resistors whose resistance is tuned by the gate voltage applied to the MOSFETs. This technique suffers from the disadvantage that large blocker signals present at the input of the active filter may force the MOSFETs that are functioning as variable resistors into a non-linear region of operation, thereby causing distortion in the signals present at the output of the active filter, as previously described.